Field of the Invention and Related Art Statement
The present invention relates to an apparatus for controlling a steer angle of front wheels of a vehicle, and more particularly to an apparatus for effecting an active control in which a steer angle of the front wheels is increased actively in accordance with a steering wheel angle.
There has been proposed an apparatus for actively controlling a steer angle of front wheels of a vehicle, in which, in addition to a usual control of the front wheels in proportion to a steering wheel angle, the steer angle of the front wheels is auxiliarily controlled in an active manner. There has been also proposed an apparatus for auxiliarily controlling both the front wheels and the rear wheels in an active manner. The latter apparatus is utilized in a so-called four wheel steering (4WS) vehicle. For instance, in Japanese Patent Publication Kokai Sho No. 60-161,266, the above mentioned steer angle controlling apparatus is described.
In the known apparatus for actively controlling the steer angle of front wheels, an active control amount for auxiliarily or actively controlling the steer angle of the front wheels is calculated in accordance with a steering wheel angle and a running velocity of the vehicle, and the steer angle of the front wheels is additionally increased by the thus calculated active control amount. This improves the stability and handling properties of the vehicle. For instance, when the steer angle of the front wheels is increased by the active control amount, upon a turn of the vehicle, a head-turning characteristic of the vehicle can be improved due to an increase in a yaw rate, i.e. an angular velocity of a yawing movement of a body of the vehicle.
In the known apparatus for controlling the steer angle of the front wheels in the active manner, the active control amount is derived by effecting calculations based on a steering wheel angle and an angular velocity of the steering wheel angle. Therefore, although the steering wheel angle is the same, a sideslip angle of the front wheels might be changed in dependence upon the angular velocity of the steering wheel angle. That is to say, the sideslip angle of the front wheels is increased in relation to a differential in time of the steering wheel angle. This results in that at an initiation of steering wheel operation by a driver, a force necessary for manipulating the steering wheel, i.e., a so-called steering force, is increased very abruptly, so that the steering force shows a large variation as illustrated by a curve A in FIG. 4C. Therefore, the known apparatuses for actively controlling the steer angle of the front wheels could not achieve optimum stability and maneuverability.